When thinking of a comet it is natural
to remember Halley's Comet because it is so famous and
usually easy to view during regular visits to Earth.
There are many other comets some that are new visitors
to our part of the Solar System. One of them is Comet
Wild 2, which did not pass close to Earth until 1974,
when the power of Jupiter's gravity changed its orbit.
It now travels much closer to the sun, between Jupiter
and the Earth. Because it has not been exposed to the
Sun at close range, its composition has not been altered
much from its original condition. By the time Stardust
encounters it, Comet Wild 2 will have made only five
trips around the sun. By contrast, Comet Halley has
passed the sun more than 100 times, coming close enough
to have been greatly altered from its original condition.
When a comet comes close enough to
the sun to get heated up, it loses some of its material
through the process of sublimation. This happens when
a solid becomes a vapor without first melting into a
liquid. After about 1,000 trips past the Sun, a comet
loses most of these volatile materials and no longer
generates a coma, which is made up of the gases that
sublime off its surface. Since it is the escaping gases
that drive the dust particles from the nucleus - the
solid part of the comet - the comet no longer creates
the long beautiful dust tail that we can sometimes see
in the night sky.
Since Wild 2 has passed the sun only a few times, it
still has most of its dust and gases and it is relatively
pristine condition. This is important because comets
are made up of material left over from the solar nebula
after the planets were formed. Unlike the planets, most
comets have not changed very much since the formation
of the solar system. Therefore, comets may hold the
key to understanding the early development of the Solar
System. Comet Wild 2 should contain much of this ancient
material, making it an ideal choice for study.
An important aspect of the journey
of Comet Wild 2 is the fact that it will be in the right
place at the right time for a visit. Particularly helpful
for the purposes of the Stardust mission is that scientists
were able to plot the spacecraft's flight path to encounter
the comet during a period of comparatively low speed.
Because of this low velocity meeting, the spacecraft
can capture comet dust, rather than having it blow right
through the collectors. The dust samples can then be
brought back to the Earth to be analyzed.
|Last updated November